The present invention relates to metal-graphite brushes containing a metal sulfide solid lubricant, which are used in automobile motors, etc., and in particular, relates to make the metal-graphite brushes substantially free of Pb.
Metal-graphite brushes have been used as brushes for low-voltage operation, such as brushes for automobile motors. Metal-graphite brushes are produced by mixing graphite and metal powder such as Cu powder, molding, and sintering the mixture. As they are operated at low voltages, their resistivities are lowered by compounding metal powder of which resistance is lower than that of graphite. A metal sulfide solid lubricant such as molybdenum disulfide or tungsten disulfide and Pb are added to metal-graphite brushes for heavy loads in most of the cases.
In recent years, Pb has been attracting greater attention as one of materials damaging to the environment, and a demand for Pb-less brushes is grown. Of course, brushes containing no Pb have been available up to the present and they have been used in some motors other than starting motors. Even some brushes for starting motors may be used by simply eliminating Pb from them, provided that they are used under normal service environments. To improve the lubricating properties without Pb, Japanese Patent Opening Hei 5-226048 (U.S. Pat. No. 5,270,504) proposes that a metal having a melting point lower than that of Cu is mixed in such a way that Cu and the metal do not form an alloy.
The inventors found that in metal-graphite brushes wherein a metal sulfide solid lubricant is added to Cu and graphite, the elimination of Pb results in an increase in the brush resistivity or an increase in the lead connection resistance under high temperature or high humidity. The above-mentioned Patent Opening Hei 5-226048 does not disclose any increase in the brush resistivity or in the lead connection resistance under high temperature or high humidity.
A primary object of the invention is to control the increase in the connection resistance of an outer terminal in a Pb-less metal-graphite brush containing a metal sulfide solid lubricant at high temperature.
A secondary object of the invention is to provide a specific structure for that.
A secondary object of the invention is to control the increase in the connection resistance of the outer terminal in high humidity as well as controlling the increase in the connection resistance of the outer terminal at high temperature.
A secondary object of the invention is to control the increase in the brush body resistivity as well as the increase in the outer terminal connection resistance at high temperature or in high humidity.
A secondary object of the invention is to obtain advantages with a small addition of Ag.
Moreover, a secondary object of the invention is to provide a production method of a metal-graphite brush which may be controlled the increase in the connection resistance of an outer terminal at high temperature.
A secondary object of the invention is to provide a production method of a metal-graphite brush which may be controlled the increase in the connection resistance of an outer terminal in high humidity as well as at high temperature.
A secondary object of the invention is to provide a production method of a metal-graphite brush which may be controlled the increase in the connection resistance of an outer terminal and the increase in the brush body resistivity at high temperature and in high humidity.
Further, an object of the invention is to provide a production method of a metal-graphite brush which may be controlled the increase in the connection resistance of an outer terminal and the increase in the brush body resistivity at high temperature and in high humidity.
In the invention, a metal-graphite brush, comprises: a Cu-graphite brush body added with a metal sulfide solid lubricant; and an outer terminal connected to the brush body, characterized in that Ag particles having a mean particle size of not more than 5 xcexcm are added to at least one of the brush body and a neighborhood of connecting interface between the brush body and the outer terminal. The added Ag particles control the increase in the resistance between the brush body and the outer terminal at high temperature.
The metal sulfide solid lubricant is, for example, molybdenum disulfide or tungsten disulfide, and its addition is, for example, 1-5 wt % of the brush body. As molybdenum disulfide and tungsten disulfide are equivalent to each other, while molybdenum disulfide is used in the embodiment, the results are identical when it is substituted with tungsten disulfide. As for the outer terminal, for example, a lead wire being molded in the brush body is used. The lead wire may be, for example, a stranded wire or a braided wire of nonplated Cu wires. In the invention, expression such as addition of Ag particles, addition of Zn powder, addition of a metal sulfide solid lubricant, or Pb-less does not refer to Ag, Zn, a metal sulfide solid lubricant, or Pb being contained as an impurity.
As it is difficult to obtain Ag particles having a mean particle size of 5 xcexcm or under from electrolytic silver, Ag particles used here are produced by chemical reduction. In the case, Ag particles are prepared by adding a reducing agent such as Zn, formalin, or ferrous ions to, for example, an aqueous solution of silver nitrate to reduce it. The kind of the reducing agent is optional, and the solvent for the solution is also optional. By the chemical reduction, Ag particles having a mean particle size of 5 xcexcm or under may be easily obtained, and the mean particle size is, for example, from 1 to 3 xcexcm. When silver nitrate is reduced by ferrous ions in the presence of, for example, citric acid, Ag black having a mean particle size of about 3-10 nm may be produced and this Ag black may be used as well. Thus, the mean particle size of chemically reduced silver is normally 3 nm-5 xcexcm, preferably 0.1-5 xcexcm, and most preferably 1-3 xcexcm. Ag particles prepared by the chemical reduction are granular, or flaky when such particles are crushed by a stamp mill. In contrast to them, the particles of electrolytic silver have normally tree-like structure. Hence, electrolytic silver particles may be distinguished from Ag particles produced by the chemical reduction by the particle structure. The mean particle size of electrolytic silver is, for example, about 30 xcexcm.
Preferably, in addition to the Ag particles, Zn is added to at least one of the brush body and the neighborhood of the connecting interface between the brush body and the outer terminal. This is effective in controlling the increase in the connection resistance of the outer terminal both at high temperature and in high humidity.
In the addition of Ag particles or Zn at least in a neighborhood of the connecting interface between the brush body and the outer terminal, preferably, each amount of the addition of Ag particles or Zn powder is 0.05-3 wt % or 2-10 wt % of the brush body material.
When Ag particles of 0.05-3 wt % of the entirety of the brush body or Zn of 2-10 wt % of the entirety of the brush body is almost homogeneously added to, for example, the brush body, the increase in the resistivity of the brush body as well as the increase in the connection resistance of the outer terminal may be controlled.
Ag particles are a precious material, and the usage of silver may be reduced by adding Ag particles and Zn only to a neighborhood of the connecting interface between the brush body and the outer terminal.
According to the invention, a production method of a metal-graphite brush having a brush body and an outer terminal, comprising a step for producing the brush body by sintering a compounded powder including graphite powder, Cu powder, and a metal sulfide solid lubricant, is characterized in that the compounded powder, to be used at least in a neighborhood of a connecting interface between the brush body and the outer terminal, further includes Ag particles produced by chemical reduction and having a mean particle size of not more than 5 xcexcm by 0.05-3 wt % based on a weight after sintering.
Preferably, the compounded powder, to be used in at least the neighborhood of the connecting interface between the brush body and the outer terminal, further concludes Zn powder by 2-10 wt % of Zn based on a weight after sintering in addition to the Ag particles.
Preferably, the entirety of the brush body contains: the Ag particles produced by the chemical reduction and having the mean particle size of not more than 5 xcexcm by 0.05-3 wt %; and the Zn powder by 2-10 wt % based on a weight after sintering.
More preferably, the compounded powder is blended to make the Zn powder disperse and contact with the Cu powder.
According to the invention, a production method of a metal-graphite brush having a brush body comprises: compounding and mixing graphite powder, a metal sulfide solid lubricant powder, Cu powder, Ag particles produced by chemical reduction and having a mean particle size of 5 xcexcm, and Zn powder to a compounded powder; molding the compounded powder; and sintering the molded powder into the brush body.
Preferably, a content of the Ag particles is 0.05-3 wt % and a content of the Zn powder is 2-10 wt % based on a weight after sintering.
According to some experiments by the inventors, it was found that when metal-graphite brushes being substantially free of Pb and containing a metal sulfide solid lubricant were exposed to high temperatures, the resulted increases in the connection resistance of the outer terminal and in the resistance of the brush body were greater than those of brushes containing Pb. It was also found that such metal-graphite brushes showed larger increases in the connection resistance of the outer terminal and in the resistance of the brush body in high humidity than those of brushes containing Pb.
According to the experiments by the inventors, the increase in the lead connection resistance and the brush body resistivity under high temperature or high humidity is attributed to the metal sulfide solid lubricant. When the metal sulfide solid lubricant was not added, the lead connection resistance and the brush body resistivity did not increase substantially even under high temperature or high humidity. This is related to the presence or absence of Pb. When Pb was added, the lead connection resistance and the brush body resistivity hardly increased in such conditions. In Pb-less brushes, in correspondence with the increase in the lead connection resistance and the brush body resistivity, the copper powder and the lead embedded in the brush body showed a greater tendency to be oxidized under high temperature or high humidity.
The metal sulfide solid lubricant such as molybdenum disulfide or tungsten disulfide is added by the designer of the brush, but the metal sulfide solid lubricant is indispensable to brushes so as to have a long service life. Without metal sulfide solid lubricant, an excessive wear may be generated. In particular, this phenomenon is conspicuous in starter brushes to which Pb has been added. When Pb and the metal sulfide solid lubricant are eliminated simultaneously, the service life of the brush will be reduced significantly. Hence in many cases, the metal sulfide solid lubricant cannot be eliminated from Pb-less brushes.
The inventors estimated the mechanism by which the metal sulfide solid lubricant accelerates the oxidization of the copper powder and the embedded lead under high temperature or high humidity as follows: At the time of sintering the brushes, sulfur is liberated from the metal sulfide solid lubricant added to the brush and sulfur adsorbs on the surface of copper to produce copper sulfide. If moisture acts on copper sulfide under high humidity, strongly acidic copper sulfate will be produced to corrode severely the copper powder and Pb. Although the behavior of copper sulfide under high temperature is not certain in some aspects, it is estimated that copper sulfide is oxidized to increase the electrical resistance.
The mechanism by which Pb prevents the oxidization of the copper powder in the brush and the embedded lead is not known exactly. The inventors estimate that Pb contained in the brush partially evaporates at the time of sintering and coats the surface of copper in the form of a very thin Pb layer. And this Pb layer protects the inner copper from sulfate ion, etc.
The inventors searched for materials which may prevent, in place of Pb, the increases in the outer terminal connection resistance and the brush body resistivity at high temperature and in high humidity. Ag particles having a mean particle size of 5 xcexcm or under were found to be effective in preventing the increases in the outer terminal connection resistance and the brush body resistivity at high temperature, and Zn was found to be effective in preventing the increases in high humidity. As Ag particles having a mean particle size of 5 xcexcm or under are added to the brush body or the connecting interface between the brush body and the outer terminal in the invention, the increase in the outer terminal connection resistance at high temperature may be controlled. It should be noted that electrolytic silver powder having a mean particle size of about 30 xcexcm, which is the silver powder used normally, could not control the increase in the outer terminal connection resistance at high temperature. Thus, to secure the function of Ag particles, it is important that the particle size of Ag particles is small.
When Zn is added in addition to Ag particles, the increase in the outer terminal connection resistance in high humidity may be controlled. The function of Zn seems to relate to the fact that Zn evaporates to coat surfaces of Cu during sintering.
When Ag particles and Zn are added only to a neighborhood of the connecting interface between the brush body and the outer terminal, the amounts of the additions may be kept low and the increase in the outer terminal connection resistance may be controlled, but the increase in the brush body resistivity cannot be controlled. In contrast to this, when Ag particles and Zn are added, for example almost homogeneously, to the brush body, both the increases in the outer terminal connection resistance and the brush body resistivity may be controlled.
It should be ensured that Zn evaporates during sintering to coat the surfaces of Cu, and it is not desirable to confine Zn in graphite powder. For example, it is preferable to fully mix graphite powder, Cu powder, a metal sulfide solid lubricant powder, Ag particles, and Zn powder to prepare a compounded powder.
To control the increases in the outer terminal connection resistance and the brush body resistivity at high temperature, it is preferable to set the concentration of Ag particles at 0.05-3 wt %, and to control the increases in the outer terminal connection resistance and the brush body resistivity in high humidity, it is preferable to set the concentration of Zn at 2-10 wt %.
Control of the oxidation due to a metal sulfide solid lubricant is particularly significant when nonplated Cu wire, which tends to be oxidized, is used as the lead wire.